Ensure graph view loads when result is clicked

Without these changes, a race condition was sometimes hit when viewing
a graph. There are two, related issues that are fixed. These problems
did not appear in the past since rendering a normal results view is
much faster and the message handler is always already set up by the
time the interface first sends a message over to the web view.

1. `vscode.postMessage({ t: 'resultViewLoaded' });` was being called
   before the component is completely mounted. Ie- `componentDidMount`
   is not called. So, the interface is notified that the web view is
   ready to receive messages _before_ it is actually ready to receive
   messages.

   The change ensures the interface only sends messages when the web
   view is ready.

2. `this._panelLoaded` is never set to false if the panel is unloaded.
   This means that if a panel is re-opened, the interface assumes that
   the view is nearly _immediately_ ready to receive messages.

   The change ensures that the interface waits for the webview to really
   be loaded before sending messages.

In both of these cases, if the interface sends the `setState` message
too early, then the message is ignored since no handlers have been added
to the web view.

extensions/ql-vscode/src/interface.ts

@@ -203,6 +203,7 @@ export class InterfaceManager extends DisposableObject {
         () => {
           this._panel = undefined;
           this._displayedQuery = undefined;
+          this._panelLoaded = false;
         },
         null,
         ctx.subscriptions

extensions/ql-vscode/src/view/results.tsx

@@ -302,6 +302,7 @@ class App extends React.Component<Record<string, never>, ResultsViewState> {
   componentDidMount(): void {
     this.vscodeMessageHandler = this.vscodeMessageHandler.bind(this);
     window.addEventListener('message', this.vscodeMessageHandler);
+    vscode.postMessage({ t: 'resultViewLoaded' });
   }
 
   componentWillUnmount(): void {
@@ -320,5 +321,3 @@ class App extends React.Component<Record<string, never>, ResultsViewState> {
 }
 
 Rdom.render(<App />, document.getElementById('root'));
-
-vscode.postMessage({ t: 'resultViewLoaded' });